Is Frequency Enough?: The Frequency Model in Vocabulary Size Testing

ABSTRACT Modern vocabulary size tests are generally based on the notion that the more frequent a word is in a language, the more likely a learner will know that word. However, this assumption has been seldom questioned in the literature concerning vocabulary size tests. Using the Vocabulary of American-English Size Test (VAST) based on the Corpus of Contemporary American English (COCA), 403 English language learners were tested on a 10% systematic random sample of the first 5,000 most frequent words from that corpus. Pearson correlation between Rasch item difficulty (the probability that test-takers will know a word) and frequency was only r = 0.50 (r2 = 0.25). This moderate correlation indicates that the frequency of a word can only predict which words are known with only a limited degree of and that other factors are also affecting the order of acquisition of vocabulary. Additionally, using vocabulary levels/bands of 1,000 words as part of the structure of vocabulary size tests is shown to be questionable as well. These findings call into question the construct validity of modern vocabulary size tests. However, future confirmatory research is necessary to comprehensively determine the degree to which frequency of words and vocabulary size of learners are related.